In a WLAN, wireless communication links are used as a transmission medium to exchange data between various stations. Due to the nature of wireless communications, it is difficult to physically detect a collision event when multiple stations transmit data frames at the same time. As a result, a typical WLAN protocol requires each frame transmission to be acknowledged by the receiver. In response to a received DATA frame, the receiver transmits an acknowledgement (ACK) frame, which indicates to the original transmitter that the DATA frame was received without errors. Accordingly, the transmitter assumes that no significant collision event happened during the DATA frame transmission. If the ACK frame is not received, the transmitter assumes that some collision event causes the DATA frame to be lost.
FIG. 1 illustrates the successful transmission of a DATA frame from Station A to Station B that acknowledges the reception by sending an ACK frame. FIG. 2 illustrates a collision event caused by simultaneous transmission of DATA frames by Station A and Station C. As a result, Station B does not receive data. Station A and Station C will assume that a collision has taken place because neither of them will receive an ACK frame from their intended receivers.
As shown in FIG. 3, Station A may retry to send its DATA frame to Station B. However, a collision with a DATA frame from Station C may again prevent Station B from receiving information. As no acknowledgement is sent, Station A must repeat its DATA frame again. This transmission may be successful, and Station B responds with an ACK frame.
A collision event caused by multiple stations competing for a shared network usually occurs at the beginning of transmission. Because the Medium Access Protocol indicates to stations on the network that the medium is free at approximately the same time, any stations with pending transmissions will begin to transmit at approximately the same time. When this occurs, the resulting transmissions will have a collision event that physically begins at or near the beginning of the transmission. However, the transmitting device is unaware of the occurrence of the collision until it has completed its attempt to transmit the DATA frame, and does not receive the expected ACK frame.
Thus, the collision event cannot be detected until the end of the transmission attempt. The longer the DATA frame, the longer it will take for the transmitting station to determine that the collision event has occurred.
Furthermore, if a collision event occurs at or near the beginning of a long DATA frame, most protocols and physical implementations render the entire DATA frame unreceivable. Therefore, the portion of the DATA frame after the beginning of the collision event wastes network bandwidth. The sooner a collision can be detected by a transmitter, the sooner the transmitter can end the faulted transmission and stop wasting network bandwidth.
To minimize the probability of a collision, some WLANs use a short request-to-send (RTS) frame sent before the actual data transmission to request an airtime reservation. For example, U.S. Pat. No. 5,231,634 to Giles et al. discloses an RTS frame sent by a transmitting station to inform other WLAN stations that the sending station wishes to use the medium for a data transmission. The RTS frame contains a time duration field which indicates to all receiving stations the amount of airtime requested for a data transmission. The requested airtime corresponds to the amount of time required to send the DATA frame plus the time required to send the ACK frame, plus the time required to receive an acknowledgement for the airtime reservation request, plus some additional small amount of time required to provide interframe spacing.
As the RTS frame itself may encounter a collision, an acknowledgement for the RTS frame is sent by a receiving station. A clear-to-send (CTS) frame may be sent in response to the RTS frame to confirm the airtime reservation. The CTS frame repeats the airtime reservation duration information.
Thus, when an RTS/CTS frame exchange mechanism is used, the following sequence of events takes place: the transmission of the RTS frame by transmitting Station A to request an airtime reservation, the transmission of the CTS frame by receiving Station B to provide an airtime reservation confirmation, the transmission of the DATA frame by Station A, and then the transmission of the ACK frame by station B to acknowledge the reception of the DATA frame.
If this frame exchange encounters a collision, it should normally occur at or near the beginning of the exchange, i.e. during the RTS or CTS frame transmission. For example, FIG. 4 illustrates two transmission attempts when the RTS frames sent from Station A were not received by Station B because of collisions with data simultaneously sent by Station C. However, the third RTS frame from Station A is received by Station B, which responds with the CTS frame. After receiving the CTS frame, Station A transmits its DATA frame acknowledged by the ACK frame sent by Station B. The airtime reservation requested by Station A and confirmed by Station B provides a time interval for the transmission of the DATA frame and ACK frame.
As the RTS and CTS frames are made short, a collision that occurs during the transmission of either of these frames results in stopping the transmission after a short time interval. Thus, the RTS/CTS frame exchange mechanism prevents network bandwidth from being wasted. Once the RTS/CTS exchange is successful, the airtime reservation information contained in the RTS and CTS frames is used by other stations in the network to avoid transmitting data during the requested airtime reservation interval. As a result, the DATA frame and ACK frame transmissions can proceed with greatly reduced possibility of encountering a collision event.
It should be noted that the RTS/CTS frame exchange mechanism requires that some of the network bandwidth be sacrificed. In a WLAN having many competing stations, the bandwidth lost to RTS/CTS frame exchanges is more than made up for by the reduction in wasted bandwidth that would have occurred due to collisions during long DATA frame transmissions. However, when the number of competing stations in the WLAN is small, the relatively infrequent long DATA frame losses due to collisions may amount to less overall network bandwidth loss than the loss due to the inclusion of the RTS/CTS frame exchange which preceded each DATA frame.
Therefore, some WLANs use a RTS/CTS threshold that specifies the minimum length of a DATA frame for which the RTS/CTS frame exchange mechanism is used. If the length of a DATA frame queued for transmission exceeds that minimum length, then RTS/CTS frame exchange is enabled for that frame transmission, otherwise the RTS/CTS exchange mechanism is not used.
In the event of the failure of a transmitting station to receive an acknowledgment frame (i.e. a CTS frame or an ACK frame), the transmitting station assumes that a collision took place. However, the transmission failure may be caused, for example, by a noise-induced bit error, or by some other interference source such as a non-WLAN transmitter. Also, the intended recipient may be out of the range of the transmitting station.
In the case of a bit error due to noise, the bit-error rate (BER) of the system is relatively fixed for a given signal/noise ratio. As this ratio changes, the BER may increase or decrease. Because of the fluid nature of the radio environment, the noise (and hence the signal/noise ratio) is constantly changing. When the a noise increases, the probability that a frame transmission experiences an error will increase.
Because of the presence of a changing noise factor, many WLANs employ yet another mechanism intended to improve their performance. That mechanism is the ability to fragment frames.
As illustrated in FIG. 5, before transmission, a larger DATA frame may be divided into smaller DATA frames sent one at a time. The receiver must re-assemble the smaller DATA frames back into the original DATA frame. Each of the smaller DATA frames is called a DATA fragment. The reason that network throughput is improved when fragmenting is performed is that when the BER is high, a long DATA frame will have a greater probability of containing at least one error. In most WLAN systems, there is no recovery from the presence of even a single bit error, and hence, the entire DATA frame must be ignored, no ACK frame will be generated by the recipient, and the transmitter must assume that a collision has taken place and it will backoff and reschedule the DATA frame transmission attempt. The unsuccessful DATA frame transmission represents lost network bandwidth. In addition, it is likely that the long DATA frame may encounter a single bit error on successive attempts, and even more bandwidth is lost.
If a DATA frame is divided into smaller fragments, only one of the fragments will contain a single bit error, and only this fragment will need to be retransmitted. Therefore, the lost network bandwidth is substantially reduced.
In the case of DATA frame fragmentation, each DATA fragment sent by transmitting Station A is acknowledged by an ACK frame produced by receiving Station B. Each DATA fragment and ACK frame contain airtime reservation duration information, which allows the station sending the DATA frame fragments to prevent other stations from data transmission during the indicated airtime reservation interval.
The DATA frame fragmentation may be used together with the RTS/CTS exchange mechanism. The possible frame exchange illustrated in FIG. 5 may consist of the RTS frame acknowledged by the CTS frame, DATA fragment #1 acknowledged by the ACK frame, DATA fragment #2 acknowledged by the ACK frame, DATA fragment #3 acknowledged by the ACK frame, etc.
The total time required to transmit a fragmented DATA frame is greater than the time required to sent a single long DATA frame, because each DATA fragment contains headers and trailer representing its physical layer and its medium access control protocol layer. Moreover, each DATA fragment is individually acknowledged, and interframe spaces exist between each fragment and ACK frame.
However, if a long DATA frame had a reasonable probability of being retransmitted, then the total time required to perform the multiple transmission attempts of the single long DATA frame could be larger than the total time required for transmission of all of the shorter DATA fragments including the additional overhead, the additional ACK frames, and the interframe spaces, even if some of the DATA fragments were retransmitted.
A typical WLAN has the ability to set a fragmentation threshold equal to the minimum length of a DATA frame subject to fragmentation. Any DATA frame having length that exceeds this fragmentation threshold is divided into fragments having length less than the fragmentation threshold.
Thus, it would be desirable to dynamically adjust the RTS/CTS and fragmentation thresholds to provide the highest possible WLAN throughput for the current operating conditions.
Also, it would be desirable to monitor WLAN parameters used for adjusting the RTS/CTS and fragmentation thresholds to maximize the WLAN throughput for the current operating conditions.